Field of the Invention
The present invention relates to a package suitable for a semiconductor device used in the microwave band and the millimeter wave band.
Background Art
A semiconductor package used in the microwave hand and the millimeter wave band has a structure in which a circuit substrate having a semiconductor element, a matching circuit, and the like mounted thereon is mounted on a planar portion in a cavity of the package, and has a cavity resonant frequency depending on dimensions (e.g., depth, width, and height) of the cavity portion (e.g., see Japanese Patent Application Publications Nos. H5-83010 and 2000-236045, Japanese Patent No. 5377096, and Japanese Patent Application Publication No. H9-148470). For simplicity, microwaves and millimeter waves will hereinafter be generically referred to as high-frequency waves. In the case where this cavity resonant frequency and an operation frequency band of the high-frequency package overlap or come close to each other, this results in undesired oscillations caused by deterioration in isolation between input and output terminals, a dip (discontinuous depression) in bandpass characteristics within the operation frequency band, a reflection gain, and the like. Thus, it becomes difficult to obtain desired high-frequency characteristics.
Generally, semiconductor packages used in the microwave band and the millimeter wave band are designed with minimum sizes from the viewpoint of size and cost reduction. In the case where a high-frequency circuit mounted in a package has a relatively low output power and a relatively low operation frequency band, the package-specific cavity resonant frequency is sufficiently high relative to the operation frequency band. However, since package dimensions have increased in accordance with recent demands for higher output power and higher frequencies, the cavity resonant frequency has decreased, and has begun to often come close to or overlap the operation frequency band of the high-frequency circuit.
Dimensions of a package are roughly determined by dimensions of a high-frequency circuit to be mounted, e.g., dimensions of a semiconductor element, and dimensions of substrates of an input/output dividing and combining circuit and a matching circuit. As an example, in packages for mounting 100 W-class output power amplifiers in the 12 GHz band, the 14 GHz band, and the 18 GHz band in which frequencies are relatively close to each other, package dimensions are roughly equal. However, since the above-described problem that the cavity resonant frequency comes close to or overlap the operation frequency band often occurs, optimum package dimensions and a cavity's internal structure must heretofore be selected for each frequency band. Thus, there has been a problem that package development periods become long or that cost reduction is inhibited.
In recent years, from the viewpoint of cost reduction, attention is focused on a trend toward increasing the replacement of metal by dielectrics typified by ceramics in part of package material. One typical example is an example in which a lid (hereinafter referred to as a cap) and a side wall of a package are made of a dielectric material (e.g., see Japanese Patent No. 5377096 and Japanese Patent Application Publication No. 119-148470). In the case where a front side or a back side (cavity side) of a dielectric cap is fully covered with a metal layer by plating or the like, an electromagnetic shielding effect can be maintained as in the case where a metal cap is used, and a high-frequency circuit mounted in the package can operate with no influence of the environment outside the package. However, a dielectric cap has a higher permittivity than air and therefore acts in the direction of decreasing the cavity resonant frequency. Accordingly, in the case where the operation frequency band and the cavity resonant frequency specific to dimensions of the package are close to each other, the dielectric cap needs to be used with particular care.
In view of the above-described technical background, some measures have heretofore been disclosed. In Japanese Patent Application Publication No. H5-83010, to cope with a decrease in a cavity resonant frequency associated with an increase in package dimensions, an electromagnetic shield wall is provided in a package to divide a cavity. As a result, dimensions of individual cavities become small, and a decrease in the cavity resonant frequency can be reduced. However, there has been a problem that the shield wall significantly limits a method of mounting a semiconductor element in a package and dimensions of a circuit substrate.
In Japanese Patent Application Publication No. 2000-236045, a method is described in which a protruding portion is provided in a central portion of a metal cap to reduce a decrease in a cavity resonant frequency. However, this method has a problem that the height of the cap increases and that machining cost for deforming the cap increases.
Japanese Patent No. 5377096 and Japanese Patent Application Publication No. H9-148470 show examples in which an opening portion is provided in part of a metal film on a cavity-side surface of a dielectric cap made of a ceramic or the like to reduce a decrease in a cavity resonant frequency caused by the dielectric cap and reduce electromagnetic coupling between a wire for mounting a semiconductor element or a circuit substrate in the package and a metal film on an inner side of the dielectric cap.
However, in Japanese Patent No. 5377096 and Japanese Patent Application Publication No. H9-148470, in the case where a metal film is provided on not only the cavity side of the dielectric cap but also a front side thereof and where the metal film is connected to conductor portions of a bottom and a side wall of the package to provide complete electromagnetic shielding in the upward direction from the package, resonances determined by dimensions of the dielectric cap newly occur, and there has been a problem that the newly occurring high-order-mode resonances enter the operation frequency band.